Reactive hot melt adhesives are typically thermosetting urethanes derived from polyols and diisocyanates which cure in the presence of moisture. Residual isocyanate in the urethane polymer mixture provides for crosslinking which takes place through moisture cure. Specifically, moisture cure affords extensions through urea formation, and crosslinking through biuret formation:
Depending upon the amount of humidity, this cure normally takes approximately 72 hours. It would be desirable to have a system which cures rapidly and is more controlled in that it does not dependent on available moisture. It would also be desirable to have a system which is applicable to “just-in-time” or “JIT” manufacture. That is, when the adhesive is applied to assemble a part, immediate cure to form a stable bond should be obtained so that the part can be shipped without delay. The adhesive should also have a pot stability for several hours at temperature typical for hot melt type adhesives, around 120° C. The present invention has found that such a system arises from using nitrile oxide precursor compounds which, when heated, form a thermally stable crosslink.
Nitrile oxides can be generated in situ by the reaction of hydroxamoyl chloride with a tertiary amine or other base. However this method can be corrosive because the CI ion in the environment will attack metal surfaces. In addition, this method of generating nitrile oxide occurs at room temperature and therefore cannot be used in a one part hot melt system.
U.S. Pat. No. 3,931,106 discloses the in situ thermal decomposition of furoxans to produce dinitrile oxides which can then be used to modify a polyfunctional species, such as in polymer crosslinking. The dinitrile oxides produced by this method cure very rapidly at temperatures in the range of 80 to 110° C.
Nitrile oxides have also been generated from precursor compounds which are produced by a method involving the generation of the potassium enolate of ethyl nitroacetate in situ using K2CO3. The carbanion reacts with the electrophile, here phenyl isocyanate in toluene. Leslie-Smith, M. G., et al., Tet. Let. 1994, 35, 9251-9254. This method however provides low yields, 30% of an aromatic precursor compound.
Therefore a need exists for aliphatic nitrile oxide precursor compounds and for faster curing of reactive hot melts which are non-moisture curing. A need also exists for a more controlled crosslinking system than is currently possible with the isocyanate moisture cure chemistry.